Turbine rotors usually have a cylindrical base body which is provided with a plurality of disklike, radially outwardly protruding portions forming blade carrier portions. These portions are disposed coaxially with the remainder of the cylindrical turbine rotor and are provided along their circumference with a number of profiled grooves extending axially or obliquely to the axial direction. The grooves secure a suitably shaped blade root; to avoid local overloads and to assure precise mounting of the respective turbine blade, they must be manufactured to precise dimensions, that is, with only a slight tolerance of 0.01 mm, for instance.
The grooves are typically produced in a milling process, of the kind described in Kollmann: "Methoden und Maschinen zur Frasbearbeitung von Generator- und Turbinenrotoren" Methods and Machines for Milling Generator and Turbine Rotors!, Publication No. 9, Kollmann Maschinenbau GmbH, 5602 Langenberg/Rheinland, Federal Republic of Germany. The appropriate blank is supported on a chucking and indexing attachment that serves to position the blank for milling of the individual grooves. Producing a groove involves multiple machining steps. In a first such machining step, the groove is cut with a disk milling cutter in a rough milling operation. This forms an undercut-free contour that is only roughly like the later or, in other words, desired groove form.
A disk milling cutter is guided through the workpiece in a feeding motion longitudinally of the groove to be cut, i.e. opened. In a second machining step, which is a step of final machining of straight or oblique grooves, a high speed, high performance steel (HSS) shaft, or shaped, milling cutter is guided with maximum precision longitudinally through the groove, in order to form the desired fir-tree-shaped groove with a dimensional error of less than 0.01 mm. In this milling machining step, on the sides of the rough-machined (pre-cut) groove, longitudinally extending undercuts are made that produce the typical fir-tree profile in cross section.
The feed of the HSS shaped milling cutter is effected in the longitudinal direction, and a relatively large amount of material must be removed, especially in the undercut regions to be formed. The attainable feed speed is intrinsically limited; values between 40 and 80 mm per minute are attained. Conversely, a certain minimum machining time for each individual groove cannot be undershot. Given the large number of grooves to be formed on the circumference of the turbine rotor, the total machining time of the turbine rotor is considerable, and this results in high cost.
Another set of problems arises in conjunction with the HSS shaft milling cutters. Their ground surface determines the attainable accuracy of the grooves in shaped milling. This means that a HSS shaft milling cutter has to be reground, before wear causes it to exhibit a pronounced dimensional deviation. A relatively large inventory of tools is therefore necessary to assure that sufficiently sharp and at least approximately dimensionally accurate tools are available at all times in the milling process, while worn-down HSS shaft milling cutters are being reground in the suitably arranging grinding facility. Moreover, in the regrinding, which is done at the chip faces, a dimensional deviation of the milling cutter is produced because of the acute-angle placement of the flanks to the circumferential direction.